As much as I would love to help anyone wanting to do custom changes, such as what you are wanting to do is really going to be up to you. I know there’s tons of tutorials out there.
All I suggest is that you always maintain a backup of your theme before doing anything custom so that if anything goes wrong, you can always revert back.
Quick note regarding what you found in a WordPress thread…that is for physically inserted images, not background images which is done from the css level. The img css you posted is actually already coded in the theme.